The present invention relates to an optical element incorporated in a bidirectional optical communication module used for transmitting and receiving terminals of bidirectional optical fiber communication of a wavelength multiplex system.
In recent years, an optical communication system using an optical fiber as a communication medium such as a high-speed Internet and an IP telephone has been widespread. This optical communication system transmits simultaneously a plurality of optical signals in different wavelengths through an optical fiber by a WDM (Wavelength Division Multiplex System) and for transmitting and receiving terminals of optical signals, a bidirectional optical communication module is used.
Such a bidirectional optical communication module is well-known, for example, by Patent Document 1 indicated below. The bidirectional optical communication module described in Patent Document 1, as shown in FIG. 5, stores a light emitting element 100 and a light receiving element 700 in a package 120 and has a holographic diffraction grating 600 installed on the top or bottom of a cover glass 220 arranged at the package opening.
In the bidirectional optical communication module, at time of transmission, a light beam emitted from the light emitting element 100 transmits through the diffraction grating 600 and a 0-degree diffracted light beam is focused by a lens 300 on a surface 500 at the end of an optical fiber 400. At time of reception, a light beam emitted from the surface 500 at the end of the optical fiber 400 reaches the diffraction grating 600 via the lens 300 and is diffracted by the diffraction grating 600, and a + primary diffracted light is focused on the light detection surface of the light receiving element 700, and a transmission signal transmitted by the optical fiber 400 is received.
The diffraction grating 600, as shown in FIG. 6, is composed of a diffraction grating of a binary structure having linear grating slits 200 formed on the cover glass 220 (refractive index n).
Patent Document 1: U.S. Pat. No. 5,555,334
The diffraction grating used as an optical element of the bidirectional optical communication module (hereinafter, may be referred to as just the diffraction grating) has a fine structure, though to prevent the diffraction efficiency from lowering and maintain the transmission factor high, it is required to prepare a highly precise grating structure. On the other hand, the diffraction grating, to produce in large quantities at a low price, is generally prepared by molding using a mold tool.
However, when preparing a diffraction grating by molding using a mold tool, since the grating structure is fine, there is a disadvantage that at time of mold release after molding, it is easily deformed. When the grating structure is deformed, the diffraction efficiency is lowered and the transmission factor is lowered.